1. Field of the Invention
The present invention relates to an electrophotographic image forming apparatus such as a printer or a digital multifunction peripheral, as well as to a control method and program for the image forming apparatus. Specifically, the present invention relates to an image forming apparatus that digitally corrects a curve and inclination of a laser beam without performing a process for optically adjusting a laser scanner unit, as well as to a control method and program for the image forming apparatus.
2. Description of the Related Art
The following method, as disclosed in Japanese Patent No. 2633877, is employed to correct a color shift among color plates in a so-called tandem color image forming apparatus. Specifically, pattern images for registration correction are formed on an intermediate transfer belt, and are read by a sensor. In such a method, a registration error amount of each of the color plates is obtained by reading the pattern images and then is fed back to the image forming processing for the each color plate. Thereby, a color shift among the color plates is corrected.
Meanwhile, a method is known for reducing costs for the electrophotographic image forming apparatus by digitally correcting a curve of a laser beam without performing a process for laser scanner adjustment. For example, such a method is implemented by using a technique disclosed in Japanese Patent No. 3388193.
For example, in the digital correction of a scan line in a sub-scanning direction, an image is formed by appropriately changing from one line to another on the basis of a curve amount of a laser beam acquired in advance, so as to offset the curve amount. The line mentioned here is a set of pixels arranged in a main-scanning direction. In addition, changing from one line to another means that, when an image is to be formed on a certain line, the image is formed by using pixels one line above or below the certain line in the sub-scanning direction, according to the curve amount. Since the curve amount is offset by performing this line change, an image in which the curve is visually corrected can be formed.
To be more specific, when a curve amount of a laser beam from a main-scan position x is expressed as f(x), a line change amount is determined as a number −y acquired from a value y which is obtained by rounding off f(x). Then, all data in sections xi to xj having an equal line change amount is shifted by −y line. When this processing is applied to the entire image region, the curve of the laser beam is offset, and an original image can be thus reproduced.
In the above processing, if f(x) from the main-scan position x is evaluated for each pixel, the digital correction can be performed with very high accuracy. More specifically, firstly, each of pixels in a main-scan direction is sequentially evaluated as to whether or not a one-line shift of the laser beam occurs from the main-scan target position in the sub-scan direction. Then, the processing for the line change is performed based on the evaluation result. In this way, an image that is accurately reproduced from the original image can be formed. However, since such processing requires a bit operation, considerable time is needed to perform the processing, pixel by pixel, on the entire image region by software. In order to reduce the time of the processing it may be necessary to prepare expensive dedicated hardware.
In practice, the curve of a laser beam f(x) from the main-scan position x is usually very small. Specifically, even in an optically-unadjusted laser scanner unit with 600 dpi, the curve f(x) of a scan line in the sub-scanning direction can be approximated to a quadratic curve in the case where the width in the main-scanning direction is 210 mm that is the short side of A4 size paper. It is well possible to manufacture the laser scanner unit so that the height of the curve of a scan line in the sub-scanning direction can be, specifically, less than 1 mm.
In the above case, an effective method is to handle the main-scan position x for every multiple pixels. For example, in the above specific example, even with a laser scanner unit having the largest curve, when the curve f(x) of the scan line is evaluated either on a pixel-by-pixel basis or on a 32-pixel basis, an evaluation error in the sub-scanning direction is about 0.5 line at the maximum for either case. This evaluation error is such a small degree that the evaluation error cannot be visually recognized in an image printed on paper. In this way, the digital correction can be performed by a logical operation for every 16 bits or 32 bits. Accordingly, the processing time for software can be shortened, and the costs for hardware can be reduced.
Further, there is another method for the digital correction. In this method, not only the curve of a laser beam but also a mechanical inclination of the laser beam are digitally corrected by use of a combination of the correction of the color shift among the color plates and the digital correction processing.
In this case, an allowable amount for correction of the color shift among the color plates is usually very similar to an amount of the line change according to inclination components. Therefore, if the pattern images for registration correction are formed with no consideration paid to the inclination components, the pattern images thus formed may exceed the range in which the color shift amount can be measured. Namely, it is supposable that the color shift cannot be corrected.
The correction may be possible by making modifications in the color shift amount measurement and operation processing for the correction. However, the processing per se then may get complicated, and several exceptional processes may also be required. To avoid this, it is desirable that the pattern images for registration correction, too, be subjected to the digital correction including the line change processing in consideration of the curve amount and mechanical inclination amount of a laser beam, and be formed on the intermediate transfer belt or the like.
Here, when a line change is performed in the formation of a pattern image, the pattern image includes a step. Thus, when the pattern images for registration correction are scanned, the measurement of the color shift of each color may include an error. Methods for avoiding this include one disclosed in Japanese Patent Publication No. 2000-253231.
However, depending on a hardware configuration, the method disclosed in Japanese Patent Publication No. 2000-253231 cannot avoid the problem of the color shift measurement error caused by the step.
The above problem might be avoided with the smoothing processing disclosed in Japanese Patent Publication No. 2000-253231 if, for example, the smoothing processing is performed by use of an engine configured of hardware capable of interpolation with a pitch less than one scan line in the sub-scanning direction, and capable of actually printing the interpolated data. However, it is quite expensive to implement such an electrophotographic engine capable of performing interpolation with the pitch less than one scan line in the sub-scanning direction, too, and of printing the interpolated data.
If the smoothing is performed by forming small dots in the main-scanning direction by means of a PWM chip, each dot can be made small in the main-scan direction but not in the sub-scanning direction. Therefore, the line width of one scan line in the sub-scanning direction can be controlled only in a unit of line. For this reason, although the smoothing can visually correct the density, the smoothing cannot solve at all the foregoing problem of the color shift measurement error of less than one line in the sub-scanning direction.
Furthermore, the accuracy for the color shift amount measurement of the pattern images for registration correction, too, may be degraded if the digital correction processing on the main-scan position is performed for every multiple pixels so as to, for example, reduce costs of the hardware, as described above in this section, Description of the Related Art. Accordingly, it is very likely that a shift amount visually unnoticeable becomes a major problem in the processing for the color shift amount measurement which tries to correct a color shift amount of each color with accuracy less than one line.